The present invention relates to a sealing unit comprising a soft sealing ring and a back-up ring which is harder than the soft sealing ring. The present invention also relates to a process for producing such sealing units.
Sealing units of this type are known in many forms and consist of separate parts. Although the dimensions and shapes of the thrust ring and sealing ring are matched to one another, they are nevertheless not connected to one another. The function of the back-up ring is to prevent a possible extrusion of the sealing ring into the gap between the machine elements. In other words, the thrust ring eliminates the danger that the sealing ring will be squeezed into the gap between the particular machine elements under an excess pressure exerted on one side.
One disadvantage of these known sealing units is that, if not fitted carefully, the sealing ring which is usually an O-ring of circular, i.e., toroidal, cross-section will possibly be inserted and inherently twisted into the receiving groove of the machine part to be equipped with the sealing unit. This reduces the sealing effect and the service life of the sealing unit. Especially where polyoxymethylene back-up rings are concerned, this can lead to rapid wear because the sealing ring is peeled off in layers.
A further disadvantage of the prior sealing units is that the known sealing units can only be used where the use was intended from the outset because of appropriate dimensions of the receiving groove. In machine parts with grooves, in which only a sealing O-ring is inserted, they cannot serve as a substitute for this O-ring since the fitting space is not sufficient for the back-ring which also has to be incorporated. This prevents the possibility of substituting a sealing unit leak-proofing means for a leak-proofing means consiting only of an O-ring, although this would often be desirable in order to increase the resistance to gap extrusion.